The present invention relates generally to rotary recording mediums. More particularly, the invention relates to a rotary recording medium of the type wherein reference signals for control of the tracking of a reproducing tracing element are recorded on opposite sides of a continuous spiral track. In the medium the reference signals are recorded in a manner such that, for example, the outer peripheral part onto which the reproducing tracing element is lowered and caused to contact can be made wide. Moreover, the reproducing tracing element can undergo displacement in a short time to the information recorded track part.
Previously, a system which records an information signal as a variation of a geometrical shape along a spiral track on a rotary recording medium, without the provision therein of a guide groove for a reproducing stylus, has been described in U.S. patent application Ser. No. 785,095, filed Apr. 6, 1977, and entitled "Information signal recording and reproducing system", and the continuation-in-part application thereof, U.S. patent application Ser. No. 33,324, filed Apr. 25, 1979, now U.S. Pat. No. 4,315,283, and entitled "Information signal recording and reproducing system".
On the recording medium (referred to as "video disc" hereinafter) are recorded an information signal such as television video signal, a first and a second pilot or reference signals for tracking control, and a third pilot or reference signal for switching the first and second reference signals at the time of reproduction. The first and second reference signals are alternately recorded at positions intermediate centerlines of adjacent information signal tracks. The third reference signal is recorded at a predetermined position on every information signal track. A reproducing system reproduces the information signal together with the third reference, and the first and second reference signals by a single reproducing element. A tracking control signal is produced from the first and second reference signals switched responsive to the third reference signal separated from the reproduced information signal. The tracking of the reproducing tracing element is controlled by this tracking control signal. The reproducing tracing element thereby traces accurately along the information signal track even without a guide groove.
In a disc of this character, the track pitch is made very small, for example, 1.4 .mu.m, in order to obtain a high recording density. For this reason, in the recording system, a high precision is required in the feed, in the disc radial direction, of the light beam for recording the information signal on the disc. Accordingly, it is very difficult to form an unrecorded portion of large width at the so-called leading part in the vicinity of the outer periphery of the disc as in a conventional audio record disc.
Therefore, in a disc of this character, the track is formed with the normal pitch up to the outer periphery of the disc. It will be supposed that the reproduction of the information signal is to commence within 5 seconds, for example, from the instant the reproducing tracing element is lowered onto the outer periphery of the disc. For this purpose, in the case where a video signal of 4 fields is recorded for every revolution of the disc, it is necessary that the reproducing tracing element be lowered onto the outer periphery of the disc within 105 .mu.m (calculated from 1.4.times.15.times.5=105) since the disc is rotated at a rotational speed of 900 revolutions per minute. As a matter of mechanical precision, it is extremely difficult to lower the reproducing tracing element accurately onto the disc in this manner within the very narrow allowable range of position in the order of merely 105 .mu.m at the start of reproduction of the disc.
On the other hand, if the allowable range for descent of the reproducing tracing element is set at a large value, the time from the lowering of the reproducing tracing element to the instant when, upon reaching the track portion where the information signal is recorded, it starts to reproduce the information signal will be long.
Thus, it is desirable that the allowable range for descent of the reproducing tracing element be large. Moreover, there is a short time interval from the instant at which the reproducing tracing element is lowered to the instant when reproduction of the information signal is started. The realization of both of these features at the same time has been desired in the art.
As one method of realizing these features, the application of the so-called random-access method known in the art appears to be possible. More specifically, the method comprises recording beforehand an address signal over the entire track inclusive of the track portion in the vicinity of the outer periphery of the disc. The reproducing apparatus reads out the difference between the address number of the track turn on which the reproducing tracing element was lowered and the address number of the initial track turn of the information signal, and quickly displaces the reproducing tracing element until the address number difference becomes zero.
By this method, however, an address signal must be recorded beforehand on the disc, and the recording system tends to become complicated. Furthermore, circuits for carrying out address detection and random access control become necessary in the reproducing apparatus, whereby the reproducing apparatus disadvantageously becomes complicated and expensive.